Please take a look at this forum post
which provides several possible solutions for the behavior you are experiencing.
Additionally, if the problem still perstit maybe the settings of the application pool got messed up resulting in very low memory limit. What is the memory consumption of the w3p process? Could you please also check the connectionstring in the DataConfig.config file found in App_Data/Sitefinity/Configurations and make sure it points to the correct database.
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